It is HTML::Form bug, report it here. Here's a workaround :)

sub HTML::Form::ImageInput::form_name_value
{
    my $self = shift;
    my $clicked = $self->{clicked};
    return unless $clicked;
    return if $self->{disabled};
    my $name = $self->{name};
    $name = (defined($name) && length($name)) ? "$name." : "";
    return ("${name}x" => $clicked->[0],
        "${name}y" => $clicked->[1],
        "$$self{name}" => $self->value,
       );
}
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